The present invention relates to a diversity receiving system applicable to digital microwave communications and, more particularly, to a space diversity receiving system which combines in-phase carrier waves received by a plurality of antennae.
In a microwave communication system, it has been customary to use an in-phase combination type space diversity receiving system in order to reduce the effects of fading and, thereby, to insure reliable communication. To combine received waves in the in-phase state, this type of receiving system includes phase control means which is disposed before a demodulator. The phase control means is made up of a low frequency oscillator, a phase shifter, a phase modulator, an amplitude detector, a synchronous detection circuit, and a control circuit, as described later in detail. The phase modulator functions to modulate the phase of one of two intermediate frequency (IF) signals by using a sinusoidal wave signal (sensing signal) which is supplied from the low frequency oscillator. The phase shifter shifts the phase of the other IF signal while being controlled by a signal which is applied thereto from the control circuit. The outputs of the phase modulator and phase shifter are combined by a combining circuit, or combiner, the resulting wave being outputted via the amplitude detector. The sensing signal, which is detected by the amplitude detector, is subjected to synchronous detection at the synchronous detector to which the output of the low frequency oscillator is coupled. The synchronous detector generates a DC signal having positive and negative polarities and representative of the deviation from an in-phase combination phase. The DC signal is fed via the control circuit to the phase shifter so as to control the same such that in-phase combination is constantly performed. A drawback inherent in such phase control means is the intricacy of construction. Another drawback is that the error rate of received signals is apt to increase since the sensing signal which is applied to the received signal is regarded as an interference component of the received signal. Furthermore, because the phase modulator and phase shifter are located in the route through which the received signal is propagated, they undesirably attenuate the received signal and lower the reliability of the communication line.
An arrangement similar to the phase control means described above is disclosed in U.S. Pat. No. 4,079,318 issued on Mar. 14, 1978.